In its broadest sense, the term "film" as used herein refers to self-supporting materials having a wide range of thicknesses. Examples would include thicknesses in the range of 0.05 to about 40 mils, more typically about 0.25 to about 5 mils (1 mil equals 1/1000 of an inch). Films can be made using a variety of techniques such as casting, blowing, and extrusion.
Good clarity in polyethylene blown film as indicated by low Haze and high Gloss has been noted in the past to be dependent upon several factors. Typically the Haze increases (and the Gloss decreases) as the polymer density and molecular weight distribution increases. Also, it has been noted that typically the surface roughness increases as the molecular weight distribution and density increases. Film stiffness on the other hand, which is often a desired property of the blown film dependent upon the actual application, has been noted to increase as density increases. Therefore, there has usually been a trade-off between film clarity and stiffness in polyethylene blown film.
Often in forming multi-layered films, a base layer of high molecular weight high density polyethylene or medium molecular weight high density weight polyethylene has been employed to provide strength and a low density polyethylene or linear low density polyethylene layer has been provided to provide other properties. Often, however, it has been noted that the low density polyethylene and linear low density polyethylene layers are tacky and sticky unless antiblock agents are included. Such antiblock agents, however, generally also have an adverse effect upon the clarity and physical properties.
An object of the present invention is to provide a method for producing films of ethylene polymers having a density of at least about 0.925 g/cc which have a good balance of processing, physical, and optical properties.
Other aspects, objects, and advantages of the present invention will be apparent from the following comments.